Recovery of cyanid from alkalin liquors



"Patented June 1 ,19 1 7 i s f f s ares?earENTO C ctas nifronw. Lawn; or riser-rues, HInALGo, MEXICO s eaves-Y or c-Yr rrrnrsorrALKALIN LIQUORS no Drawing. Application filed December-l4, 1922. serial nseam-59. f

I v vention relates totherccovery of values. Eachinine or. Working, ofcourse, cyanid trom alkalin solutions thereoh'nnd it? present'slocal problems, but inageneral way,

v has particular reference to a method. ofreitnray be said that the cyanid liquors bcoveringicyanid in a form suit-able for use tainedatte'r leaching the ore and removing This i ii anew by purifying andstabilizing the. liquor thefricher values, contain essentially,cyanid' with the controlled addition of acid, and orCy.) combined withalkali (potassiuln thereafter recovering the purified liquor. and sodium, etc.) and also some other metallic In the metallurgical arts the disposition cyaiiids', suchjas silver andcopperfcyanids. of weak or exhausted cyanid solutions result These various cyanide exist in jcomplexrela- Mingfrom the treatment or ores "is a technical t'ionships, the nature of Which need 'not be problem.- Forhmany years' it has been the; hereinfrirther set forth for a full understandusualpractice, in treating auriferous or aring of the instant invention. It is suflicient gentiterous ores for therecoveryof gold, silto notethat they do exist, Without 'specula-I v r, and other values, to leach the ore with a tlon upon the various matters of theory dilute solutionof an alkalin cyanid, such for: offered in explanation thereof] \35 example, as solution of potassium, sodium, The 'cyanids are salts of hydrocyanic acid or calcium cyanid. The leach liquor is then (prussic acid) and are extremely poisonous. treated for'the removal of its precious metal Hydrocyanic'acid, itself,;(I-I CN) is a deadly contents. Ordinarily the pregnant liquor poison, reacting with Water through'hydrolagitated with zinc dust is pumped through ysis to form other nitrogenouscompounds, filter presses or the pregnant'solution is such as urea, with, the partial liberation of passed throughboxes containing zinc shavcyanogen (CN) The hydrolysis persists ings. In thistreatment the gold'and silver even when the solutionis madealkalin and,

and other metals more electropositive than in practice, it-is customary to add an excess zinc Tare precipitated While the latter "goes of alkali, usua lly'i'n the formoflime, to'reinto solution. f dnce this source of lossto a minimums lllore recently, the cyanid extraction proc-f In treating ores containing sulfids there are ess hasbe'en proposed for use 'in connection also formed sulphocyanates, which also reprewith the treatment of ores other than gold .sent a source of loss. Metals,'likevvise, corn- 3'11! and silver, specifically, copper and mercury. binewith the dissolved cyanid existing in Where the precious metal contents of such I the leaching liquid, and, unless these metals ores'are 'srna llin comparison with the base areirecovered ortheir combinations with metal -contents"the' loss of the latterinthe cyanid broken down, such combinations also oyanid' solution;represents a large money represent a loss of cyanid from the process 3 value/ The base metals dissolved in the eye Another loss occurs throughthefcustomarv ni'dsolutionusually represent considerable process of filtering the pulp obtainedinv value, liere recoverable, and the cyanid treating the ore? The/ et pulpf(filter cake) combined'vvith' these metals isoften sogre'at i contains some cyanid and unless extensive that its-value, When'not liberated from the Washing is resorted to this] retained fraction V base metals and recovered.constitutes a loss of the solution also'lost. In practice, is greater than the value of thegold and silver. not feasibleto' Wash the filter cake to too'great recovered. Under such conditions the ore in an extent "since, the washings must, to be questionj'could not be treatedat a 'profit; "saved, enter-the millingsolutioncircuitand Whereas, if the base metals were recovered, i the latter must be maintained'more or less I 45 as Well as the cyanid combined with them, constant in volume, which would be impos 5 such an ore could be commercially utilized. sible'with unlimited Washing. Owing to the losses of base metals, as Well Accordingtothis invention, there isproas'o cyanid, when treating impure gold or, videda process for recoveringlcyanid from silver fores, considerable attention has been liquors,.such as those justreferred to,'with directed-to the problemof recovering these economies in operations and theavoidance of" 1" In. another way of the cyanid as hydrocyanic acid or lime sullitein such cases.

tion is readily processed for the removal of itsjmetallic and cyanid contents.

In one way of treating such liquors here- .tofore proposed the solution was treated with sulfuric acid to neutralize the solution. With such treatment, the complex alkalin metallic cyanide were said to be converted into simple cyanids, thereby liberating in the solution one half of the cyanid so combined. The half quantity of cyanid was then recovered by separating the precipitate of metalliccyanid from the liquor, and then adding an excess of lime to the residual solution to precepitate'. the sulfate as calcium sulfate, providing thereby an alkalin cyanid solution for use in treating fresh quantities of ore. lThis process is obviously not sulficiently far reaching for modern practice, since only one half of the available cyanid is recovered, and such small, but valuable amounts of metals as are retained in the solution,are not recovered. a

working, the alkalin cyanid solutions have been treated with an excess of acid, suchcas sulfurous acid, by means of which the lime present is partly precipitated as calcium sulfite and the resulting unstableacid solution is then treated with air, with or without heat, or the like, to drive off cyanogen for absorption in an alkalin medium. The addition of excess acid has, however, the disadvantageous efli'ects ofre-dissolving some of the precipitate ofcalcium sulfite, and necessitating the use ofexcessive amounts of acid. It is necessary in practice to employ extensive apparatus, and to guard carefully against theescape of poisonous hydrogen cyanidor I cyanogenvapors into the surrounding atmospher'ea V V 7 In driving .off the unstable cyanid from such acidified solutions, there is invariably formed a new precipitate of calciumsulfite which deposits as a hard scale in the solution pipe lines, dispersmgtanks, filters, etc, thereby increasing maintenance costs.

to reduce clogging of the 7 acid. solutions containing re-dissolved calcium sulfite and metallic sulphocyanates there is invariably a loss of silver and copper in .theprecipitated calcium sulfite formed. Theaddition of acid to such solutions as contain retained calcium salts is, of course, also excessive and uneconomical, as these salts react with the acid, thereby consuming more than thatrequired for the simple liberation of the cyanid, and increasing the ratio of total (present in the solution as hydrate or cyanid) after which the solusuch substantially neutral maximum or complete precipitation of cal- An exces-' sive amount of S0 is also required in order apparatus with In such highly acidity to the S0 ion. Further sources of loss in the excess acid process of recovery need not be enumerated here.

According to this invention, the complex cyanids are broken up with recovery of ey anid, and the lime is precipitated by the addition of a controlled amount of acid,.usually sulfur dioxid gas (when this gas is dissolved in water it is known as sulfurous acid), or sulfuric acid. A suflicient amount of acid is added to'bring the alkalin liquor to substan tial neutrality against phenolphthalein. Vhile this is the optimum operating condi-' tion, it will of course be understood that the solution after treatment with acid may still be'slightly acid or slightly alkalin within the spirit of the invention. Neutrality is advantageous, however, as I have found that in liquors there is a cium hydrate, (and a partial precipitation of the lime present as chlorid) as calcium sulfite, and the resulting solution is so stable as regards the free escape of cyanogen as to permit of easy and safe handling. As previously noted, the addition of either insufficient, or excess acid results in either the incomplete precipitation of the calcium, or in its redissolution,

The addition of the acid toneutralize the solution is advantageously spread over a period of time, and the S0 gas is brought into contact with the solution in such a way that local excesses are avoided. In one way of operating, the alkalin cyanid liquor is sprayed through theitop of a long covered wooden box, while the S0 gas is bubbled up through the pool of solution that collects on the bottom on its passage through the box. The neutralized solution is then run into settling -or thickening tanks of such size as to permitof the retention of the liquor therein from two to four hours.

Two such tanks connected in series may be employed. In thesetanks the calcium sulfite settles in a readily separable granular form, and it may be removed as a sludge from the bottom from time to time or pumped out continuously, while the stable neutral solution may be removed from the settling tanks by providing a simple overflow, thus employing a process of decantation; or the neutral liquor'may be filtered to free it from any suspended precipitates that may be carried over with the overflowing solution. p

1 The stable solution of cyanid is thus freed from its lime content before acidification proper, and hence, the difliculties heretofore .experienced in the gradual re-pre'cipitation of lime during and following the acidifica 'r tion, in the conveying pipes:and dispersioii; apparatus, Will not 1 he encountered; The purified solution,.,-free from lime and contain 1 ing available cyanid in stable form, and also silverand copper and other values, is then, conducted into-acidifying apparatus Wl1 8I- incontrolled additional quantities of S0 .or other'acid, are introduced to liberate the cyanid from the solution as hydrocjyanic acid or cyanogen, and also to "condition the solu- 7 tion' for removal of recoverable metallic critical relationship exists' between the S0 7 Proper conditions obtain hen-the soluperienced." Accordingly,

the'liberation of the-cyanid combined-with values. It has'b'een discoveredi'that afsomevvhat acidioni contentpf the bath and the total acidity, favorable to-the "I11&Xi111l1 m"- 1GCOVe1y of 'cyanid andtherecoverable metallic contents remaining" therein: Depending upon the specific 'coin'p'ositionflof the liquorundergoing treatment, jtherei ore, the amount of" acid employed inthesecoiid phase of addition isxso controlled or adjusted as to favor the metals, in orderfthat it may be recovered, and 'tfurther', properly to prepare the solution for the maximu n recov'ery ot-the gold, silver, copper, and other metals.

acidis employed, only-a slight excessof S05 need be introducedin the second phase of the liquor undergoing treatment.

process to produce a definite acidity'in' the Such acidity is best measured against methyl orange as an indicator instead of phenolphthalein, as in the simultaneously with the precipitation bfthe previous case. In. the presence of suihcient free S0 and soluble sulphocyanates, the silver and copper present in the'solution are completely precipitated as insoluble sulphocyanates, providing that enough of the cyanid in the solution has, been driven'oti. The precipitated metals can be settled out as 7 "a sludge, or filtered. V The cyanid with which these metals were originally combined is retamed in the solution-for recovery later, Or

inetals'a I V p The step of adding more S0 to unstabilize .th neutral liquor and'so render the conta ned,

cyanid available for removal may herein-i after be referred to as acidification, hile the operation ofactually removing the ey anid iii the form of'a gas for subsequent ah 1 neutral solutionciila ti sor'ptioii in an: alkalin -solution ma be re '7 erred I to as l The dispersio s: effected by adding to the c 7 1 eenrenio'ved int-he manner previouslyoutined, enough"SO to provide the statedratio of o'tal acidity-t0 free SO 7 ion, then placing this. i

pressure air through it, "when the cyanid is decomposed and'goes oiflas. gas and ispassed' throughjun" alka-hn absorbing solution WlllCh iiizes' the ga es-a stablea'lkalin 'cyanid;v

' absolute ratio of, total {acidity to" free desired I condition under which complex cyanids Will be brokend'ownso asto liberate cyanidfina v gaseous form."--HoWever,'too high an acidity I .L l

tends to tardjth'e speed ofthedispersion'reaction; it

absorbent.

aded with circulating cyanobecomes 0 erlo Sen or a hadly. Thatis,

poor absorption causes poor dispersion and consequently, low cyanid re coverie's; Low total acidity with high free 'SO ion content are the conditions sought, in

that these 'conditions vill'give the highest dispersion andabsorption eii'iciencies and the greatest recoveries of silver, copper,

and such conditions vcan only be .re-:

anid

{from Which -the jlime has properly ac ified'solution under partial vacuum or passing-large volumes of low n wvi llfvaryivi'th the composition ofthe dispersed, but a low-total acidity free, SQ "-acidityrepresents the I condition. The acidity indicates a and cydispersion of the solution.

besides I neutralizin a ,lar er j V quantity oraiirali in the absorbing solution, 1 "thus reduciu the e fiic ency of the latter-as an With poorabsorption'thej 13011 I l i cn-absorption system l the dispersion thenifalls as i Y alizcd where the acid solution being dispersed -is free 'fromliine s ilfites and'bisuliites. w

'Due'to the previous elimination of tllGllHlS hydrate,- the ratio of total acidity toit'reeSO on iseasil y maintained at the proper point,

he nearer this ratio approaches unitywith thetotal acidity "kept to thesminimum consis-tent Witl 'mai" mum dispe rsion t he more nearly the conditionsof acidityapproach the ideal for-the best results; ";Under such conditiojus, there isobtainedan increasedvyield of cyaiiid and metallic values With'a lower con-f sumption' of $9 "The conditions for the naxiiinuni dispersion'and the recovery of the cya'nid are hencebbtaihable whenworkingin this Way,-or statingthe matter in other lan-. v

' guage :afgreater'recovery of cyanid'; less total cyanid loss less acid consumption ;..less corrosive 'actionvon' the apparatus (due tolow. 'aciditi'es)-;'=less maintenance'cost to keep the 1313111531416 om lime sulfite deposits; and a? are provided by the 'present'procedurey The slightly acid solution," from 'vVhi hthe 7 lime was-removed'by a preliminary neutrali ;zation W thSQ containing unstable cyanid,

may *be dispersed at lnormal temperatures or reduction ot loss-in residual metal-recovery,

it may, before entering the first disperserybe i the hydrogen cyanid heatedlfrom say, 80 F:'., or lower, to 1109 F3 T his increase in' temperature-is desirable as and cyanogen is evolved" practically twice as fast at 1105 F. as at 80 Then treating solutions free'from lime, as are ere obtained, the liquor may be heated by steam coils passing into the apparatus, or by any other suitable means. This is not feasible when'the solution contains lime as the latter would form a hard scale 'on the coilsor other heating elements, thus preventing efiicient heat transfer. Many installationsusing a cyanid recovery process need operate in cold climates where the application'of heat to the acid solution previous to dispersion is essential toeconomic success, and for such instal-r lations, the present process is particularly adapted.

' In working up the metallic precipitatesob tained after final acidification and dispersion,

to recover silver, copper,etc., the precipitates 7 maybe filteredto removeas much of the solution aspossible from them. The filtrate may then be discarded, or used as a' filter wash,

either with or without previous treatment with quick lime to remove the zinc as hydrate which'may be recovered. r here the lime is not removed from the solution to bedispersed, by a preliminary neutralization with S0 asdescribed in this ap- V plication, the filtration ofthe dispersed solution for the removal of the metallic precipitates encounters difficulties in that the lime gradually crystallizes out in thepores of the canvas filter leaf and even on the inner side of the leaf. v The deposit of sulfite so formed '7 is difficult to remove, especially when the lime sulfite becomes mixed with more or less of the copper and silver sulphocyanates being filtered. Such deposits cut downthe porosity of the filter leaf which necessitates considerable labor and'expensefor muriatic acid-- in order to'keep the filter leaves up to their highest efliciency. In the present process of preliminary elimination of the limein the 'acidifierfeed no such cloggingor chokingup of filter leaves with'calcium sulfite can take place, because the amount'of calcium sulfite dissolved in the liquor is too small to be thrown out of solution due to a decrease in solution acidity or a partial evaporation.

' With no lime deposits forming on the filter leaves much expense 1s saved 1n labor and materials for acid treating the'leaves and for occasionally replacing them completely with new ones;

While the invention herein disclosed has efurdioxid to precipitate the copper and any residual silver values, and to provide suflicient total acidity and free S0 to liberate hydrogen cyanid.

2. A method of recovering cyanogens from alkalin solutions of cyanides containing a soluble alkalin earth salt, which comprises neutralizing the solution with an acid capale of forming an insoluble precipitate of the alkalinea'rth, removing the precipitate, adding an excess ofacid to the solution andblowingit with air to remove thefree HUN.

3. A method of recovering cynaogens from' alkalin solutions of cyanides containing a soluble alkalin .ea-rth salt, :which comprises neutralizing the solution with a material contaming an acidion capable of forming an in-- soluble precipitate of alkalin earth, removr ng the precipitate, adding an excess of acid 1on contalnlng material and blowingit with air to remove free cyanogens.

4. A method of recovering cyanogens from alkalin solutions of cyanides containing a soluble calcium salt, which comprises neutralizing the solution with the additionof sulfur dioxid to form a precipitate of calcium sul'phite, removing the precipitate, and

then adding an-excess of sulfur'dioxid to the solution, and blowing the latter with air to remove free cyanogens.

a In witness :whereof, I have hereuntosigned my name.

. CLARENCE w. Lawn.

been described'with particular relation toits application to liquids obtained in the cyanid treatment of gold and silver ores, it will-be understood that it is not so limited, but that it may be applied to a variety of solutions. 7

I claim Y 1-. Amethod of recovering silver, Copper, 7 I

and other values from alkalin cyanid liquors i 

